Despite the significant opportunities to transform the way that organizations conduct trading activities, few studies have investigated the impetus for organizational strategic moves toward business-to-business (B2B) electronic marketplaces. Drawing on transaction cost theory and institutional theory, this paper identifies two groups of factors--efficiency- and legitimacy-oriented factors, respectively--that can influence organizational buyers' initial adoption of, and the level of participation in, B2B e-marketplaces. The effects of these factors on initial adoption of and participation level in B2B e-marketplaces are empirically tested with data collected, respectively, from 98 potential adopter and 85 current adopter organizations. The results of a partial least squares analysis of the data indicate that the two groups of factors exhibit different patterns in explaining initial adoption in the preadoption period and participation level in the postadoption period. Specifically, all three of the efficiency-oriented factors investigated in this study--product characteristics, demand uncertainty, and market volatility--and their subconstructs exhibit a significant influence on adoption intent or participation level, or both. The results demonstrate that two legitimacy-oriented factors--mimetic pressures and normative pressures--and their subconstructs have a significant impact on adoption intent, but not on participation level. Our findings also indicate that clearly different patterns exist between the two groups of factors in explaining adoption intent and participation level.